Atomic Structure and Theory

Atomic Models and Theories Overview

• The evolution of the conception of the atom can be traced
• Dalton's modern atomic theory can be explained
• The different atomic models and the experiments that determined subatomic particles can be described

Early Atomic Ideas

• Early atomic ideas originated from philosophers
• Thales, Anaximenes, and Heraclitus proposed "primal" matter made up matter
• Aristotle proposed materials are composed of four elements: fire, water, air, and earth, in different amounts
• Democritus and his teacher Leucippus proposed matter is composed of tiny, indivisible particles
• "Atomos" means indivisible matter, serving as basis for the modern term "atom"
• "Atomos" were believed to have different forms and combinations, giving rise to different materials

Dalton's Atomic Theory

• All matter is made of atoms
• All atoms of a given element are identical in mass and properties
• Atoms of a given element are different from those of any other element
• Compounds are combinations of two or more different types of atoms
• A chemical reaction is a rearrangement of atoms

Robert Millikan and the Electron

• Robert Millikan determined the electrical charge of the electron
• Millikan's "Oil Drop Experiment" balanced a bubble up while gravity pulls the bubble down
• The Plum Pudding Model of the atom remained popular

Plum Pudding Model

• J.J. Thomson suggested the existence of another particle to balance the negative particles
• Proposed that this particle is a large positive sphere with embedded negative particles
• The atom is a uniform positive bread with negatively charged plums/raisins embedded in it

Ernest Rutherford Atomic Model

• The Gold Foil experiment revealed atoms consist mostly of empty space and contain a positively charged nucleus
• Rutherford's model suggests atoms are unstable

Rutherford's Experiment Findings

• Most alpha particles passed through the foil, indicating atoms are mostly empty space
• Few alpha particles bent at approximately 1°, consistent with Thomson's model to some extent
• Some alpha particles were bent at significant angles or went back, thus Thomson's model became inconsistent
• Rutherford proposed a small, dense, positive region called nucleus
• Rutherford then discovered protons a positively charged particle in the nucleus
• James Chadwick discovered neutrons, which are neutral particles

Rutherford's Nuclear Model

• Positively charged particles are concentrated in the nucleus
• The nucleus is very small in comparison to the overall size of the atom
• Most of the atom is empty space
• Very small, negatively charged electrons move around the nucleus

About Atoms

• Atoms are the building blocks of matter, composed of protons, electrons, and neutrons
• Atoms have a nucleus containing protons and neutrons
• Atoms are mostly empty space
• Electrons are contained in shells surrounding the nucleus
• Protons have a positive charge, electrons have a negative charge, and neutrons are neutral

Niels Bohr Planetary Model of the Atom

• Rutherford's model failed to explain why electrons do not fall into the positive nucleus
• Also, James Maxwell had proven that charged particles continually emit energy, eventually the atom could no longer exist
• Niels Bohr proposed his planetary model of the atom, improving the nuclear model
• Bohr used Max Planck's theory on quantized energy of electrons
• Electrons do not need to emit energy while moving around the nucleus
• Electrons exist in a stable ground state with fixed energy
• Electrons have restricted orbits around the nucleus
• Electrons become excited when they absorb energy, moving to a higher energy level
• When electrons go back to their ground state, they release energy as heat or light

Valence Electrons

• Each electron shell can hold a certain number of electrons
  • Shell 1: 2 electrons
  • Shell 2: 8 electrons
  • Shell 3: 8 electrons
  • Shell 4: 18 electrons
  • Shell 5: 18 electrons
  • Shell 6: 32 electrons
  • Shell 7: 32 electrons
• Electron shells are filled from the inside out
• Noble Gases have full outer electron shells
• All other elements have partially filled outer electron shells
• Valence electrons are electrons in the outermost shell
• The valence shell is furthest from the nucleus

Modern Atomic Model

• Atoms consist of protons, electrons, and neutrons
• All atoms have negatively charged electrons that move around the nucleus
• Electrons move around the nucleus approx. 10^-8 cm from the nucleus
• Electrons are attracted to the positive particles in the nucleus
• Atoms have a dense region called the nucleus
• Positively charged protons and neutral neutrons reside together in the nucleus, held by a strong nuclear force
• Protons and electrons have the same magnitude of charge, but opposite signs
• Neutrons have roughly the same mass as protons
• 1,836 electrons are needed to equal the mass of one proton
• A neutral atom contains an equal number of protons and electrons, net electrical charge is therefore zero
• The average diameter of an atom is around 100-500 picometers, or 1-5 angstroms
• A nucleus diameter is approximately 1 x 10^-4 angstroms

Atomic Number and Mass

• Rutherford's experiment helped investigate the nucleus
• The mass of an atom is concentrated in its nucleus
• Positive charge results from the presence of protons
• The number of protons determine the element
• The number of electrons equals the number of protons
• An ion is formed removing or adding electrons to a neutral atom
• Cations are positive ions, and formed when an electron is removed
• Anions are negative ions, and formed when an electron is added
• Atoms of different elements have different numbers of protons and atomic numbers
• No two elements have the same atomic number
• The mass number is the sum of protons and neutrons (symbol A)
• Atoms of the same element may have the same atomic number, but differing atomic mass
• The atoms are called isotopes

Bohr - Sommerfeld Model

• Niels Bohr created a visual model of the atom to make them easy to understand
• A Bohr model constains a central nucleus surrounded by electron shells
• Arnold Johannes Wilhelm Sommerfeld expanded the idea of Bohr's Model

Quantum Mechanical Model of an Atom

• The quantum model of the atom can be described
• The behavior of electrons can be described using quantum numbers and electron configuration

Stability in Atoms

• Noble gasses are usually unreactive because they have full valence shells
• An element with a full valence shell is said to be happy
• Two atoms must gain, lose or share electrons to join together
• Elements with full valence shells do not easily gain or lose electrons
• Atoms will try to gain or lose electrons to have a full valence shell to gain stability
• Metals try to lose electrons
• Non-Metals try to gain electrons

Atomic Orbitals

• S Orbital (sharp): spherical cloud that becomes less dense with distance from the nucleus
• P Orbital (principal): dumbbell shaped cloud having two lobes on opposite sides
• D Orbital (diffused): four leaf clover shaped cloud
• F Orbital (fundamental): difficult to represent and too complex

Electron Configuration

• The electron configuration of various molecules can be written using the diagram
• S can hold a maximum number of electrons of 2
• P can hold a maximum number of electrons of 6
• D can hold a maximum number of electrons of 10
• F can hold a maximum number of electrons of 14
• Pauli's Exclusion Principle- no more than 2 electrons in an atom can occupy and orbital
• Hund's Rule- one electron must enter first in each orbital, then a second electron will be added
• Aufbau Principle- electron fill first the orbital of the lowest energy until any added electron